Don't get into a --max-snapshots infinite loop

[seanh]

When --max-snapshots is used and _remove_oldest_snapshot() doesn't actually reduce the number of snapshots reported to _ls_snapshots() (for example when --dry-run is used so that we don't actually run the rm command) then snapshotter gets into an infinite loop as it keeps calling _remove_oldest_snapshot() until _ls_snapshots() reports less than --max-snapshots snapshots, but _ls_snapshots() always reports the same number.